State-of-the-art semiconductor deposition reactors may incorporate laser-based optical systems for monitoring the particulate levels in the exhaust gas flow. However, such particle monitors are interactive with the gas flows which they monitor in that various reaction byproducts, non-reacted species, etc., deposit on and etch and, thus, optically degrade surfaces which come in contact with the gas flow. For example, deposition on the particle monitor lenses and other surfaces in and around the associated laser beam scatter laser light into the collection optics, causing noise in the photodiode or other detector. As a consequence, the reactor must be taken out of service frequently, so that surfaces such as particle monitor windows can be cleaned or replaced.